The present invention relates to a device for the surface emission of infrared radiation, with catalytic combustion of a mixture of a combustible gas (a gaseous hydrocarbon such as butane or natural gas) with an oxidizer gas, for example air, which may or may not be overpressurized or compressed.
Such a device may be used by itself, or grouped or joined together with other devices, for example a drying oven or tunnel, or in various applications, for example drying, curing of a coating, etc.
The term xe2x80x9csurface emissionxe2x80x9d should be understood to mean a surface emitting, at every point, infrared radiation, of between 2 xcexcm and 10 xcexcm, with a calorific emission power distributed homogeneously and uniformly over the entire emitting surface. Preferably, this surface is plane, for example with a polygonal outline, especially rectangular. However, such a surface may be convex or concave.
According to document U.S. Pat. No. 3,291,187, a device for the surface emission of infrared radiation is already known, this comprising a combustion chamber having a catalytic combustion wall, with a certain thickness, made of refractory material, permeable through its thickness, and suitable, for example in terms of pressure drop, for the passage of a mixture to be burnt. The latter, as in any burner, consists of a mixture of a combustible gas, for example a gaseous hydrocarbon, such as butane or natural gas, and an oxidizer gas, for example atmospheric air, and said mixture may or may not be compressed. A diffuser for the mixture to be burnt is optionally placed upstream of the heat exchange wall, so as to homogenize the mixture to be burnt before it reaches the upstream face of said catalytic combustion wall. The mixture to be burnt passes from the upstream face to the downstream face of the catalytic combustion wall, the internal developed surface of which is coated with a catalytic combustion material, for example one or more metals or metal oxides having the property of catalyzing the oxidation of hydrocarbons, for example platinum and/or palladium. This catalytic combustion chamber includes an upstream chamber in which the diffuser, supplied on one side with the mixture to be burnt and closed on the other side by the catalytic combustion wall, is optionally placed.
For devices as described above, in which the surface area for infrared emission, corresponding to the downstream face of the catalytic combustion wall, is relatively large, two difficulties are encountered in practice.
The first relates to controlling the calorific power emitted by the catalytic combustion wall, by varying the pressure of the combustible gas. In practice, for a minimum pressure of said gas, corresponding to a minimum emitted power, if the pressure drop caused by the mixture to be burnt passing through the catalytic combustion wall is too low, the developed internal surface of this same wall cools too quickly, which, in certain cases, results in deactivation of the catalytic material. In such cases, it is then necessary to re-ignite the catalytic combustion in order to return the catalytic combustion wall to its rated emission power.
The second difficulty relates to keeping the temperature at all points on the surface emitting infrared radiation high enough to keep the catalytic material activated, and thus to obtain catalytic combustion uniformly distributed over the entire aforementioned surface without a xe2x80x9cdead zonexe2x80x9d, that is to say a zone which is inactive in terms of combustion.
The object of the present invention is to remedy the aforementioned drawbacks.
More specifically, the subject of the invention is a solution particularly (but not exclusively) designed for devices having a relatively large surface area emitting infrared radiation, making it possible to limit or prevent inactivation of the catalytic material, on the one hand locally, and on the other hand when the device operates at its minimum power, when the latter is controlled or regulated.
According to the present invention, the combustion chamber includes at least one heat exchanger, placed upstream of and transversely with respect to the catalytic combustion wall. This heat exchanger is made of a refractory material, for example a ceramic, and is permeable, like the catalytic combustion wall, through its thickness, by being suitable, for example in terms of pressure drop, for passage of the mixture to be burnt. This heat exchanger forms with the catalytic combustion wall an intermediate gap, separated by the heat exchanger from the rest of the device or the upstream chamber. This heat exchanger receives, via its downstream face, directly facing the catalytic combustion wall, mainly by radiation, at least most of the heat radiated by the upstream face of the catalytic combustion wall.
Preferably, the combustion chamber includes a diffuser for the mixture to be burnt, placed, for example, in the upstream chamber, upstream of the heat exchanger.
According to the present invention, the term xe2x80x9cheat exchangerxe2x80x9d should therefore be understood to mean an element as defined above, in which no catalytic combustion takes place and which can, according to the embodiment adopted, be likened to a permeable wall having a certain thickness, through which the mixture to be burnt passes over its entire working cross section. The first function of this exchanger is to absorb at least some of the heat emitted by the upstream face of the catalytic combustion wall and to deliver it, at least partly, to the mixture to be burnt which is passing through it, immediately before it reaches the upstream face of the catalytic combustion wall.
In order for heat exchange to be effective, the heat exchanger in question is dimensioned in terms of thickness (that is to say of length in the direction of flow of the mixture to be burnt) so that the residence time within said exchanger of the mixture to be burnt is at least 0.1 s. If the thickness of the heat exchanger is too low, to the point that the residence time of the mixture to be burnt is less than 0.1 s, there is, firstly, overheating of the upstream face of the heat exchanger, at a relatively high calorific power, which might cause a safety problem, and, secondly, there would be cooling of the upstream face of the catalytic combustion wall, at a relatively low calorific power, which might inhibit the catalytic reaction.
In practice, such a heat exchanger makes it possible to effectively increase the temperature of the mixture to be burnt, from the upstream face (where said mixture is at a temperature close to room temperature) to the downstream face (where said mixture is at a temperature close to that of the catalytic combustion wall) of said heat exchanger. This rise in temperature is at least 500xc2x0 C., and preferably between 500 and 1000xc2x0 C.
In practice, the refractory material used for the heat exchanger must ensure, throughout its mass, that there is a certain level of conduction of the heat radiatively absorbed by the downstream face of said exchanger. However, this conduction must remain limited so as not to raise the mixture to be burnt to the ignition temperature before it reaches the catalytic combustion wall. Advantageously, the material of which the heat exchanger is made is a ceramic, for example cordierite. The thickness of the heat exchanger, that is to say its dimension in the direction in which the mixture to be burnt passes or flows is at least 5 cm.
The pressure drop caused by the mixture to be burnt passing through the heat exchanger must not be too great. In general, it is of the same order as that caused by the mixture to be burnt passing through the catalytic combustion wall, for example about 0.60 Pa.
According to the present invention, a functional gap, devoid of any material, between the heat exchanger and the catalytic combustion wall is more specifically between the downstream face of the heat exchanger and the upstream face of the catalytic combustion wall. Preferably, this functional gap is at most 5 mm, and is between 4 and 6 mm, so as to place the downstream face of the heat exchanger in direct view of the upstream face of the combustion wall and in the zone of maximum radiation of the latter.
Consequently, a heat exchanger according to the present invention should not be confused with a diffuser or heat shield, on the one hand because such a diffuser or heat shield may exist according to the present invention, in addition to and upstream of the heat exchanger, and, on the other hand, because in general a diffuser or heat shield, which is often relatively thin, plays practically no role in transferring heat toward the gas or the gas mixture which passes through it.
In addition, by virtue of the invention the thermal energy of the combustion not radiated by the downstream face of the catalytic combustion wall is largely stopped by the heat exchanger and returned toward the catalytic combustion wall, via the mixture to be burnt, serving as a heat-transfer medium. Thus, this arrangement avoids dissipating, toward the upstream part of the device with respect to the direction of flow of the mixture to be burnt, a major part of the heat not radiated to the outside of the same device.
Preferably, the combustion chamber includes a complementary thermal insulation wall placed upstream of and facing the upstream face of the heat exchanger, this thermal insulation wall, also made of refractory material, also being permeable through its thickness and suitable for passage of the mixture to be burnt.
This arrangement increases the relative thermal insulation between, on one side, the catalytic combustion wall and, on the other side, the upstream part of the device, thereby avoiding or limiting the phenomenon of catching fire in the upstream chamber through which the mixture to be burnt flows.
Preferably, the combustion chamber includes a chamber for distributing the mixture to be burnt, upstream of the so-called upstream chamber, as previously in the direction of flow of the mixture to be burnt. This distribution chamber is separated from the upstream chamber by a partition in which a multiplicity of expansion passages for the mixture to be burnt are distributed.
This arrangement makes it possible, in particular, to obtain stoichiometric oxidation or combustion, over almost the entire surface area of the downstream face of the catalytic combustion wall, whatever the size of the latter.